1. Field of the Invention
The present invention relates to an image forming system (or image forming apparatus) within which a process catridge can be mounted. Such an image forming system may be embodied, for example, as an electrophotographic copying machine, a laser beam printer, a facsimile machine, a word processor or the like.
2. Related Background Art
In image forming systems such as copying machines, a latent image is formed by selectively exposing a uniformly charged image bearing member. The latent image is then visualized with toner and the toner image is transferred onto a recording sheet, thereby forming an image on the recording sheet. In such image forming systems, whenever the toner is used up, it must be replenished. The toner replenishing operation is not only troublesome, but also often causes contamination of the surroundings. Further, users are inconvenienced by the fact that the maintenance of various elements or members can be performed only by an expert in the art.
To eliminate such drawbacks and inconvenience, an image forming system wherein part such as a developing device in which the toner therein has been used up or an image bearing member in which a service life thereof has expired, can easily be exchanged, thereby facilitating the maintenance, by assembling the image bearing member, a charger, the developing device and a cleaning device integrally as a process cartridge which can be removably mounted within the image forming system has been proposed and put into practical use, for example, as disclosed U.S. Pat. Nos. 3,985,436, 4,500,195, 4,540,268 and 4,627,701.
In conventional process cartridges, a developing sleeve and sleeve bearings for rotatably supporting the developing sleeve are mounted within the process cartridge as developing means. The developing sleeve is rotated during the developing operation. In this respect, a rotational driving force is transmitted to the developing sleeve via meshed gears. In this case, it is feared that the sleeve is displaced due to a force acting toward a meshing pressure angle between the gears. The displacement of the sleeve greatly affects the image quality. To avoid this, the sleeve bearings have been made stronger, or have been reinforced by a frame.
Further, in the past, in a process cartridge comprising a photosensitive drum and developing sleeve for holding toner used to form an image on the photosensitive drum, the following method was used for driving the process cartridge by a drive gear of an image forming system.
That is to say, the drive gear is positioned in such a manner that a force component F1 generated from the driving force between the drive gear and a sleeve gear and a direction of the meshing pressure between the gears is directed downwardly with respect to a horizontal direction by increasing an angle .alpha. between a line interconnecting centers of the drive gear and a flange gear of the photosensitive drum and a vertical line. As a result, the force component F1 is supported by a drum supporting member abutted against a lower surface of a drum shaft. Further, since the drum shaft is supported by the drum supporting member, a moment M is generated due to the reaction force against the driving force. The moment M urges the process cartridge against an abutment portion with a force F1. Now, since the force F2 is directed downwardly, it can be supported by the drum supporting member, as well as the force component F1.
Seals are contacted with a peripheral surface of the developing sleeve to prevent the toner from leaking from the sleeve. Accordingly, great force is required to rotate the developing sleeve, which tends to displace the sleeve. Thus, in order to prevent the displacement of the developing sleeve, the rigidity of the sleeve bearing itself and the rigidity of the frame for reinforcing the sleeve must be considerably increased, thus making the process cartridge and accordingly the image forming system large-sized.
Further, when the drive gear and the flange gear are meshed with each other so that the force component F1 is directed downwardly, it is difficult to withdraw the process cartridge upwardly due to the meshing direction between the gears. Thus, the process cartridge must be withdrawn or removed along an upward oblique direction or be withdrawn after the drive gear has been retarded, thereby making the image forming system large-sized. Furthermore, it is feared that the developing gear is displaced by a force directed toward a direction of a meshing pressure angle between the flange gear and the sleeve gear.
Incidentally, U.S. Pat. No. 5,028,966 (particularly, FIGS. 15-17) discloses the following arrangement.
That is to say, a drum gear of a photosensitive drum and a drive gear of an image forming system are arranged in such a manner that a line interconnecting rotational centers of the drum gear and of the drive gear is inclined in a counter-clockwise direction by an angle .alpha. with respect to a vertical line. On the other hand, one end of a receiving surface for receiving a drum shaft of the photosensitive drum extends slightly above a horizontal line passing through a center of the drum shaft in a condition that the drum shaft is received by the receiving surface, with the result that the one portion of the receiving surface supports the drum shaft while contacting the shaft drum the bottom to an upper portion of the latter. Further, a flank recessed portion is provided at the other end of the receiving surface to facilitate the mounting/dismounting of the photosensitive drum with respect to a housing. With this arrangement, it is possible to facilitate the mounting/dismounting of the photosensitive drum with respect to the housing and accurately hold the photosensitive drum at a predetermined position in the receiving surface.